Recently, there has been increasing interest in energy storage technologies. As the application fields of energy storage technologies have been extended to mobile phones, camcorders, notebook computers and even electric cars, efforts have increasingly been made towards the research and development of electrochemical devices. Under these circumstances, secondary batteries capable of repeatedly charging and discharging, in particular, have attracted considerable attention as the most promising electrochemical devices. In recent years, extensive research and development has been conducted to design new electrodes and batteries for the purpose of improving capacity density and specific energy of the batteries.
Many secondary batteries are currently available. Lithium secondary batteries developed in the early 1990's have drawn particular attention due to their advantages of higher operating voltages and much higher energy densities than conventional aqueous electrolyte-based batteries such as Ni-MH batteries, Ni—Cd batteries, and H2SO4—Pb batteries. However, such lithium ion batteries suffer from safety problems, such as fire or explosion, when encountered with the use of organic electrolytes and are disadvantageously complicated to fabricate. In attempts to overcome the disadvantages of lithium ion batteries, lithium ion polymer batteries have been developed as next-generation batteries. More research is still urgently needed to improve the relatively low capacities and insufficient low-temperature discharge capacities of lithium ion polymer batteries in comparison with lithium ion batteries.
Many companies have produced a variety of electrochemical devices with different safety characteristics. It is very important to evaluate and ensure the safety of such electrochemical devices. The most important consideration for safety is that operational failure or malfunction of electrochemical devices should not cause injury to users. For this purpose, regulatory guidelines strictly restrict potential dangers (such as fire and smoke emission) of electrochemical devices. Overheating of an electrochemical device may cause thermal runaway or a puncture of a separator may pose an increased risk of explosion. In particular, porous polyolefin substrates commonly used as separators for electrochemical devices undergo severe thermal shrinkage at a temperature of 100° C. or higher in view of their material characteristics and production processes including elongation. This thermal shrinkage behavior may cause short circuits between a cathode and an anode.
In order to solve the above safety problems of electrochemical devices, a separator including a highly porous substrate and a porous coating layer formed on at least one surface of the porous substrate wherein the porous coating layer is formed by coating with a mixture of inorganic particles and a binder polymer has been proposed. For example, Korean Unexamined Patent Publication No. 2007-0019958 discloses a technique related to a separator including a porous substrate and a porous coating layer formed on the porous substrate wherein the porous coating layer is composed of a mixture of inorganic particles and a binder polymer.
When such a separator including a porous coating layer employs a non-woven fabric as a porous substrate, due to the presence of large pores in the non-woven fabric, charging failure or leakage current is caused which leads to a problem of a longer constant voltage (CV) region. Further, when a pressure is applied to the separator in the fabrication process of a battery, the porous coating layer is pushed into the non-woven fabric through the large pores of the non-woven fabric, causing the above problems to become more serious. In the meantime, separators including porous coating layers are required to have a shutdown function in order to ensure improved stability of batteries against thermal runaway.